Powder metallurgy



June 1963 R. E. RICE ET AL 3,386,821

POWDER METALLURGY Filed April 26, 1967 OOOOOO 7///Y// //j /,{oodoood1 United States Patent 3,386,821 POWDER METALLURGY Richard E. Rice, Arlington, and George Warren Webb, Revere, Mass, assignors to Cornstock & Wescott, Inc, Cambridge, Mass., a corporation of Massachusetts Filed Apr. 26, 1967, Ser. No. 633,781 6 Claims. (Cl. 75226) ABSTRACT OF THE DISCLOSURE Method of making articles of powdered iron which involves only a single pressing at moderate temperature with inexpensive powder containing no carbon, quick quenching and no sintering.

In the conventional processes of making metal parts by powder metallurgy methods, it is usual to compact the metal powder in a pressing operation carried out at room temperature. In order to strengthen the parts, they arethen subjected to a sintering operation at a high temperature in a protective atmosphere for an extended period of time. Following this they may be further compressed in a coining or sizing operation to further densify them and to improve the uniformity of dimensional tolerances. In order to improve the mechanical strengths of the parts additions of alloying elements may be used; for example, a few percent of copper powder is often introduced and during sintering temperatures sufiicient to melt the copper powder may be employed. Since copper has a relatively high melting point, 198l F., this requires a costly furnacing operation. Another method by which compacts may be strengthened is through the use of an infiltration operation which involves heating a porous iron compact in contact with a carefully metered quantity of copper. The temperature is sufiicient to melt the copper and allow it to permeate the porous iron structure and to partially alloy with the iron. All of these operations are essentially batch type. Equipment for sintering is expensive, requires considerable floor space and involves relatively high maintenance costs. Processes which involve two pressing operations ordinarily require two complete sets of tooling which adds appreciably to the piece cost.

The objects of the invention are to achieve a powder metallurgy process which is continuous, which can be completed in a single piece of equipment, which requires relatively low temperatures and short times, which requires no sintering operation of the conventional type, which requires a relatively very small floor area, which eliminates in-process inventories of partially completed parts, which produces high densities with relatively low-pressure presses, which permits the use of the lowest priced powders, and which results in improved overall manufacturing economy.

According to this invention the method comprises compressing iron powder to form a compacted article and then quenching the article, characterized in that the powder is compressed while at a temperature of the order of 1100" F. to 1300 F. and the article is quenched quickly while it retains the metastable quality existing at said temperature, whereby strong articles of high density may be produced quickly with only one pressing operation and with inexpensive powder which does not contain carbon. Preferably the aforesaid temperature is approximately 1200 F. The die in which the powder is compressed should be preheated to the aforesaid temperature. While the powder may be heated in the die it is preferably preheated to the aforesaid temperature before introduction into the die. The powder may be poured into the die or it may be precompacted to form a briquette.

3,386,821 Patented June 4, 1968 A special feature of the invention resides in the timing of three critical operations of the method, carried out in sequence, namely heating the powder, its compaction, and its cooling. The invention involves the discovery that with ferrous metal composed of powder particles which will pass through a 20 mesh sieve and within the powder particles a crystal size which is customarily referred to as fine grain, the superior mechanical properties which are associated with fine grain size can be preserved to a worth-while degree through the hot pressing operation in spite of the fact that the pressing temperature is in the range ordinarily used for annealing the ferrous metal. While it is customary to anneal metal powders prior to compaction when they are to be pressed cold and even when they are to be hot pressed, in practicing this invention such annealing is to be avoided since it causes crystal growth and leads to less desirable mechanical properties in the finished article.

By pressing within the aforesaid temperature range the compacted articles have very superior qualities, such as fine grain, high density and exceptional strength. Quick quenching preserves the metastable quality existing at the pressing temperature and prevents the fine metal crystals from growing.

The invention also involves the discovery that the superior qualities can be obtained without mixing carbon with the iron; heretofore it has been thought that the presence of carbon is necessary in the quenching operation.

For the purpose of illustration a typical example of the invention is illustrated diagrammatically in the accompanying drawing in which:

FIG. 1 is a vertical section showing the parts in position to charge the die;

FIG. 2 is a plan view of the die from the line 22 of FIG. 1;

FIG. 3 is a view like FIG. 1 showing the parts in compressing position; and

FIG. 4 is a similar section showing the parts in ejecting position.

The particular embodiment of the invention chosen for the purpose of illustration comprises a die 1 such as described and claimed in copending application Ser. No. 633,780, new pending, filed on even date herewith, the die consisting of two sections held together by a wire winding 2. The die is surrounded by a housing 3 having a removable bottom 4. The die is mounted on the bottom 4 by means of a casing 6 held in place by screws 7. Between the casing and housing is a cylindrical space for a heating coil 8. The die has an axial opening 9 in which the powder is compacted by plungers 11 and 12 extending through sealed openings in the housing. With the parts in the position shown in FIG. 1 the die is charged with powder from hopper 13 by means of a screw 14 extending through a tube 16 sealed in an opening in the side of the housing. Surrounding the tube is a heating coil 17 for heating the powder to the pressing temperature or nearly thereto. The powder is guided into the die by chute 18. After the die is charged with the desired amount of powder the plunger 11 is advanced from the position shown in FIG. 1 to the position shown in FIG. 3 to form the compacted article A. Then plunger 11 is retracted and the plunger 12 is advanced to the position shown in FIG. 4 to eject the article from the die, after which the article is discharged from the housing 3 by the following mechanism.

Sliding over the top of the die is a pusher 19 actuated by a cylinder 21 and a piston connected to the pusher by a piston rod 22. When fluid is admitted to the cylinder the pusher is advanced from the position shown in FIGS. 1 and 3 to the position shown in FIG. 4, thereby depositing the article A on a belt 23 which carries the article to 'a pool of quenching liquid in a tank 24 which forms a part of housing 3 and which is sealed from the outside atmosphere. The articles are removed from the pool by a screw conveyor 26. The housing and tank are preferably kept full of an inert gas through an inlet 27 and an outlet 28.

From the foregoing it will be understood that this method is characterized by the fact that it employs only a single hot-pressing operation at a moderately elevated temperature and moderate pressure and does not require a sintering operation. Thus it makes possible a unique continuous single-step manufacturing process which has substantial advantages over the conventional multi-step processes. A uniqueness of the new process resides in the fact that the entire operation, from raw metal powder to finished article, is carried out in a very short time. The transit time of material through the process is of the order of seconds rather than hours as in conventional powder metal processing. It thus becomes possible to produce finished powder metal parts by a continuous process within 'a single piece of equipment of moderate size. Ordinary inexpensive commercial iron powders can be used to produce parts of excellent mechanical properties, and very superior mechanical properties can be obtained by the use of the somewhat more expensive electrolytic iron powders. Also from the foregoing it will be evident that articles may be formed continually by repeating the aforesaid steps of charging, compressing and ejecting. It will also be evident that the articles may be quenched quickly while they retain the metastable quality existing at the compression temperature.

It should be understood that the present disclosure is for the purpose of illustration only and that this invention includes all modifications and equivalents which fall within the scope of the appended claims.

We claim:

1. In the art of forming articles by hot-pressing iron powder to form a compact and then quenching the compact, the method which comprises compressing the powder while at a temperature of the order of 1100 F. to 1300" F. and then quenching the compact quickly to retain the metastable quality existing at said temperature, whereby strong articles of high density may be produced quickly with only one pressing operation and with inexpensive powder which does not contain carbon.

References Cited UNITED STATES PATENTS 4/1941 Boegehold 752l4 X 2/1944 Goetzel 75-227 X BENJAMIN R. PADGETT, Primary Examiner.

A. J. STEINER, Assistant Examiner. 

